Cl-xmlspam
[common-lisp.net], an "XML Stream PAttern Matcher", (also known as xspam) is a
library adjunct to cxml to allow simple on-the-fly parsing of streaming XML
based on a non-backtracking regular-expression-like parser. It
provides a compact syntax for matching, borrowing terminology from
RELAX-NG, and also for
regular-expression-based parsing of text elements, inspired by Rob
Pike's structural regular
expressions.

Some preliminary documentation can be found
here,
and one or two examples
here.

Cl-xmlspam is currently in a young state - although it has been used to
do useful work on real-world XML data, it has so far only been tested under
SBCL, and the interface is subject to change.
Feedback
is much appreciated.